This invention relates to a magnetic recording and reproducing apparatus and a magnetic recording medium. More particularly, the invention relates to a magnetic recording and reproducing apparatus for recording and reproducing signals as magnetic information on and from a disk-shaped magnetic recording medium, and a magnetic recording medium suitable for use in the apparatus.
Recently, hard disks are enhanced in density at the rate of nearly twice a year. Problems involved in this movement are thermal agitation of media, field detecting sensitivity of heads, and positioning accuracy of heads. Among these problems, thermal turbulence of media is considered solvable to a certain extent by replacing the parallel-to-plane magnetization recording system with the vertical magnetization recording system. As for field detecting sensitivity of heads, significant improvement of sensitivity is expected by using new magnetoresistance elements such as TMR (tunneling magnetoresistance effect elements).
As for the head-positioning servo technique, however, no hopeful measures coping with narrowed tracks have been envisioned heretofore. For HDD (hard disk drive) with superdensity higher than 400 Gbpsi (bit per square inch), desired head-positioning accuracy for servo operation is 5 to 10% of the track pitch. For 1 Tbpsi class, however, the desired accuracy is in the order of several nanometers. This level of accuracy is extremely difficult to achieve.
One of major reasons relates to arms natural vibration and sampling period. Nevertheless, in regard to the former, a double piggyback system, active dumping system, etc. are in the process of examination, and some way of solution will be shortly found for the countermeasure to arm natural vibration. Thus the most important issue to be contemplated for enhancing the performance of the servo system is the latter issue, namely, the sampling period.
There are “servo-plane servo system” and “sector servo system”, for example, as the servo system of a hard disk. In case of typical HDD not having so many disks, discrete type control called sector servo system is often employed.
FIG. 14 is a conceptual diagram of a part of tracks and header portion formed on a hard disk. A plurality of tracks T for recording data extend substantially in parallel at a track pitch TP, and a header portion H is provided at their forefront portion in each sector.
In the header portion H, signals called “burst signals” for obtaining track error information (position error signals, abbreviated PES) are recorded. This portion for recording the burst signals includes alternately appearing regions H1 and H2, the former having the alternate record of the “0” level and the “1” level and the latter having the record of “0” level only as illustrated.
The regions H1 are positioned offset from each other such that the centerline C of the track T passes the center of the region H1 or one of its perimeters, respectively. This offset positioning of the regions H1 relative to centerlines C of the tracks T enables its use as servo information for the tracks T.
These burst signals in the header portion H are written on the medium before shipment with a device called “servo track writer”.
A magnetic detecting region RH detects these burst signals when running through the header portion. These burst signals are processed, and a tracking error PES is extracted. That is, PES can be obtained only at the moment immediately after the magnetic detecting region RH passes the header portion H where the burst signals are formed.
When the medium disk has N sectors and the disk rotational speed is f (Hz), the servo information is controlled with the sampling period Nf (Hz). In servo control, however, the sampling period of the servo information is a constraint that restricts the servo bandwidth, and the servo bandwidth cannot be higher than approximately ⅛ of the sampling period empirically. This means that the sampling period must be increased to enhance the servo performance such as the response characteristic. Actually, there is also the constraint from arm natural vibration in addition to the constraint from the sampling period. Herein, however, it is targeted to improve the servo performance by increasing the sampling period.
Representative methods for increasing the sampling period include a method of increasing sectors formed on the medium disk and a method of increasing the rotational speed of the medium disk.
The later method, however, may invite an increase of “swinging rotation” with the increase of the rotational speed. When the disk “roughs” under high-speed rotation, disturbance itself to be depressed increases even if the disturbance-depressing rate is increased by expansion of the servo bandwidth. Therefore, residual deviation of the tracking error does not decrease but may rather grow.
On the other hand, increasing the sectors is certainly an effective method in terms of the servo performance. However, it increases the ratio of the servo area surface relative to the disk surface, i.e. the area rate of the header portion. Actually, the rate of occupation by the servo area surface is determined at the trade-off between the ratio of the servo area surface and the servo performance, but it is undesirable that the servo area surface increases to and beyond 10%. In this case, the ratio of the disk surface occupied by the header portion is too large, and the data region decreases. This problem is serious in case of the 1-Tbpsi class surface recording density, and there is the demand for a method capable of increasing the sampling period without increasing sectors.
There are some proposals directed to such requirement. For example, Japanese Patent Laid-Open Publication No. H06-215322 discloses a continuous servo system using two reproducing elements located in alignment along the width direction of a track T to detect a record signal from their sum signal and detect a tracking error signal from their difference signal.
There is also a proposal using a single reproducing head while using a special disk configured to record servo signals on the sidewall of its data region, separate reproduced signals by frequency and thereby separately acquire track error signals and record signals as disclosed in Japanese Patent Laid-Open Publication No. 2000-195200.
Any of these techniques, however, invites deterioration of S/N of reproduced signals when it is applied to HDD. A prospect of improving the sensitivity of the head upon detecting a magnetic field lies in taking measures for continuous servo with the current accessibility as a prerequisite. If the S/N ratio of the disk record signal degrades, circumstances basically change. Decreasing the leak magnetic field from the disk recording magnetic field to the reproducing head or increasing the noise magnetic field from any adjacent track, etc. makes it impossible to keep sufficient S/N of reproduced signals.
In case of the method disclosed in Japanese Patent Laid-Open Publication No. H06-215322, if those two aligned reproducing elements are brought into access to the disk, an electrode between these reproducing elements comes to a central portion of the magnetic domain of the signal recorded on the track, that is, on the center line of the track, and it therefore decreases the intensity of the leak magnetic field from the disk record to the reproducing element portion. In other words, it results in locating the electrode between the pair of reproducing elements in the central portion where the magnetic field profile of the signal recorded on the track is highest, and the read loss is large.
In case of Japanese Patent Laid-Open Publication No. 2000-195200, there is room for improvement in terms of the sensitivity on the part of the servo signals. More specifically, in case no errors are produced, the head falls in a condition no error signal is produced. Therefore, there is substantially no detection sensitivity of servo information near zero error. As a result, the servo rigidity may relatively decreases. Its reason lies in that the error information contained in the detection signal itself is very small in comparison with the signal information and S/N of the extracted error signal is bad accordingly. Thus it is difficult to increase the servo rigidity. Additionally, also in the record/reproduce signal, since the servo signals from opposite sides of the recording track contain harmonic components, even after bandwidth separation, the harmonic components affect the record signal. If there are errors, they may decrease S/N of the reproduced signal.
As explained above, for realizing superdense HDD, conventional techniques require continuous servo capable of simultaneously detecting error signals and reproduced signals with sufficient sensitivity over a sufficient bandwidth, but deterioration of S/N ratio of the reproducing head and other problems make it difficult to realize superdense HDD.